The GPS is a space-based radio positioning network for providing users, equipped with suitable receivers, highly accurate positional information. Developed by the United States Department of Defense (DOD), the space-based portion of the GPS comprises a constellation of GPS satellites in non-geosynchronous orbits around the earth. By measuring the distances between a GPS receiver and the GPS satellites, the position of the GPS receiver may be accurately determined. The process of determining a position using a single GPS receiver is generally known as absolute positioning or autonomous positioning. Under ordinary circumstances, autonomous positioning can generate results having an accuracy of approximately 100 meters in a horizontal plane when selective availability (a signal degradation deliberately introduced by the DOD) is turned on, and approximately 10 to 30 meters in the horizontal direction when selective availability is turned off.
Autonomous positioning, however, is typically not accurate enough to be used for surveying and mapping purposes. Rather, a method called differential positioning, or differential GPS positioning (DGPS), has been applied in the fields of surveying and cartography to generate more accurate positional information. In one form of DGPS, two GPS receivers are used. One GPS receiver is placed over a point such as a National Geodetic Survey (NGS) survey monument whose position is precisely known. This GPS receiver is usually referred to as the base receiver. The second GPS receiver, generally referred to as the "rover," collects data from unknown points. GPS data collected by the base receiver and by the rover are then processed to generate a relative position of the rover to the base receiver. Because the exact location of the base receiver is already known, the position of the rover can then be calculated with precision and accuracy.
Typically, after a set of GPS field data has been collected at a survey site, it is downloaded to a computer for post-collection processing. Several types of differential correction schemes are currently available. Prior Art FIG. 1 is a hierarchical diagram 100 showing the various types of differential correction schemes. Specifically, DGPS correction schemes can be classified into two categories: a code-based correction scheme 140 and a carrier phase correction scheme 150. Carrier phase correction scheme 150 can be further classified into fixed integer correction 160 with centimeter-level accuracy, and floating point correction 170 with decimeter-level accuracy. Depending on the level of accuracy desired for a particular application, a user can configure a GPS receiver and data collector computer to collect GPS field data that is appropriate for the respective correction scheme so that the GPS field data can then be processed accordingly in an attempt to derive a solution (corrected position) of the desired accuracy.
After a set of GPS field data has been collected at a survey site, post-processing is typically performed using an office-based computer. Each of the three DGPS correction schemes described above has a corresponding software processing engine for processing the GPS field data to generate a solution of the respective accuracy. Because a particular correction scheme cannot produce a solution of the desired accuracy unless the collected field data contains sufficient information, a user must know and understand the contents of the GPS field data in order to select a proper processing engine for use with that particular set of data. This can be a significant inconvenience to the user responsible for the post-processing of GPS field data. For instance, the user may not be the same person who collected the field data and thus may not know about the conditions under which the data is collected. Even if the same user is responsible for both collecting and post-processing of the data, the user may not have a detailed understanding of the collected data to identify and apply the proper processing engine to the particular set of data.
Furthermore, while GPS field data collected specifically for DGPS purposes can always be processed by code-based correction to obtain a solution of meter-level accuracy, such GPS field data may or may not contain sufficient information to be processed by carrier phase correction to obtain a meaningful solution, depending on the conditions in effect during data collection. For instance, certain types of field data receivers have limited data collection capability and may not be able to collect adequate information for carrier phase correction. Another important factor is the duration of satellite lock. If the period of continuous tracking of relevant data from one or more GPS satellites is not long enough, the field data collected may not be sufficient for carrier phase correction. Moreover, the base line length, or the proximity of the field data receiver to a base station, may also affect whether the field data collected can be processed by carrier phase correction. These and other factors will determine whether a carrier phase solution can be derived from the GPS field data collected. In other words, non-ideal conditions during the data collection session may preclude a user from obtaining a solution of the desired accuracy.
In addition, even if a user understands the contents of a set of field data, the user may not be able to determine whether a particular correction scheme would yield a meaningful solution. Often, a user has to process the available data with the fixed-integer correction processing engine and then examine the solution and also the quality of the solution, as reflected by various statistical accuracy indicators, to decide whether the fixed integer solution is acceptable. If this attempt with the fixed integer correction scheme fails, the user needs to process the data all over again with the floating point correction processing engine. Furthermore, the floating point correction processing engine usually has a user interface which is different from that of the fixed integer correction processing engine. The user must then examine the floating point solution and its quality. If the floating point solution is still unsatisfactory, the user must resort to the code-based correction processing engine and repeat the correction processing step to get a code-based solution. During this manual post-processing procedure, the user frequently has to run multiple correction processing engines and deal with their different user interfaces, wasting much time in the effort.
Moreover, a user must be able to read and interpret different solutions and their quality (based on their respective accuracy indicators) generated by the various processing engines in order to determine whether a particular solution is acceptable or not. This means a user must possess detailed knowledge of the GPS and computer data processing fields to perform the post-processing task. Additionally, this trial and error approach not only discourages many novice users but also frustrates sophisticated users, and is a great disincentive for potential users to purchase and use GPS equipment.
Thus, a need exists for a system and method for post-processing of GPS field data which does not require a user to know and understand the contents of the GPS field data. A further need exists for a system and method for post-processing of GPS field data which does not require a user to manually and sequentially invoke a series of different processing schemes in order to obtain a DGPS solution of acceptable accuracy from the GPS field data. Yet another need exists for a system and method for post-processing of GPS field data which does not require a user to deal with multiple user interfaces and different processing requirements. Still another need exists for a system and method for post-processing of GPS field data which does not require a user to read and interpret different solutions generated by various processing schemes. Additionally, a need exists for a system and method for post-processing of GPS field data which does not require a user to possess detailed knowledge of the GPS and computer data processing fields in order to obtain a DGPS solution of acceptable accuracy from the GPS field data.